1. Field of the Invention
The present invention relates to a wavelength tunable light source equipment, more particularly relates to a wavelength tunable light source equipment which can stabilize laser oscillation and freely adjust an intensity of light output with suppressed spontaneous emitted light ingredient.
2. Description of the Related Art
In a wavelength tunable light source equipment using an external resonator, the outputted laser light has been produced in a gain medium provided in the external resonator, which is for determining gain in laser resonance. The light outputted from the wavelength tunable light source equipment includes laser light of the desired wavelength and, also, spontaneous emitted light generated from the gain medium. Therefore, in the conventional wavelength tunable light source equipment, in order to output the laser light with the suppressed spontaneous emitted light, generally a wavelength selection structure and beam splitter have been arranged in the external resonator. In this conventional equipment, reflection light (or diffraction light) from the wavelength selection structure has been obtained by the beam splitter as the output light.
An example of the conventional wavelength tunable light source equipment suppressing the spontaneous emitted light will be explained next referring to FIG. 23. In a wavelength tunable light source equipment 201, a semiconductor laser diode (LD) 202 is used as the light source. The semiconductor laser diode 202 has two end faces 203 and 204. These end faces 203 and 204 are designated as the A-end 203 and the B-end 204. The A-end 203 has a reflectance of tens of percent and forms one end of a laser resonator included in the external resonator light source 205. The B-end 204 is coated with an anti-reflection film (AR film) 206. The B-end 204 has the structure for causing light not to be reflected.
The light emitted from the B-end 204 of the semiconductor laser diode 202 is converted into parallel light by a lens 207, passes through a non-polarization beam splitter (BS) 208, and enters a diffraction grating 209 of a wavelength selection structure 221. In FIG. 23, a plate-shaped diffraction grating 209 is viewed from the side. In the wavelength selection structure 221, the diffraction grating 209 is attached so as to be able to rotate about a rotary shaft 210 by a rotation drive (not shown). The diffraction grating 209 diffracts the light converted to parallel light by the lens 207. Due to the action of the diffraction grating 209, only light of the desired wavelength is emitted at the same angle as the angle of incidence to the diffraction grating 209 and is introduced again to the B-end 204 of the semiconductor laser diode 202 through the lens 207. The diffraction grating 209 functions as one end of a laser resonator in the external resonator light source 205 together with the above A-end 203.
The non-polarization beam splitter 208 is arranged as explained above in the light path between the semiconductor laser diode 202 and the diffraction grating 209. The non-polarization beam splitter 208 splits a part of the light introduced again to the semiconductor laser diode 202 from the diffraction grating 209. The lens 211 focuses the light split by the non-polarization beam splitter 208 and introduces it to an optical fiber 212. This optical fiber 212 forms a light output unit of the wavelength tunable light source equipment 201. A first light output A of the wavelength tunable light source equipment 201 is taken out from the optical fiber 212.
Here, the light output of the wavelength tunable light source equipment 201 will be explained.
The light L2 advancing from the semiconductor laser diode 202 to the diffraction grating 209 includes a spontaneous emitted light ingredient emitted from the semiconductor laser diode 202 in addition to the laser light of the desired wavelength λ1. On the other hand, the light L1 taken out from the diffraction grating 209 through the non-polarization beam splitter 208 to the side of optical fiber 212 is only the laser light of the desired wavelength λ1 in which the spontaneous emitted light ingredient is suppressed by the wavelength dispersion action of the diffraction grating 209. Therefore, the diffraction grating 209 functions as a wavelength selection structure 203. The spectral distribution of the light L1 is shown as the light output A as indicated by the reference F2 in FIG. 23. Further, the spectral distribution of the light L2 is shown as the light output B as indicated by the reference F1 in FIG. 23.
The above light L1 is outputted as the first light output A from the optical fiber 212. The output of the optical fiber 212 becomes the first light output of only the laser light of the desired wavelength λ1 with the suppressed spontaneous emitted light by the wavelength dispersion action of the diffraction grating 209. The light L1 suppressed for the spontaneous emitted light by the wavelength dispersion action of the diffraction grating 209 as mentioned above becomes the first output light of the wavelength tunable light source equipment 201. In this sense, the diffraction grating 209 forms the wavelength selection mechanism 221.
Further, the light including the spontaneous emitted light ingredient emitted from the A-end 203 of the semiconductor laser diode 202 is the same as the above light L2 with regard to the wavelength ingredient. The light emitted from the A-end 203 is converted to parallel light by the lens 213 and focused at the optical fiber 216 by the lens 215 through an isolator 214. The output from the optical fiber 216 becomes the second light output of the wavelength tunable light source equipment 201. The second light output is the above light output B. The light output from the optical fiber 216 is the light L2 and becomes the second output light of the wavelength tunable light source equipment 201.
Note that the isolator 214 is provided so as to prevent the laser oscillation of the external resonator light source 205 from becoming unstable due to light from the outside striking the external resonator light source 205 formed by the A-end 203 and the diffraction grating 209. Further, though not shown in FIG. 23, an isolator may be similarly arranged between the non-polarization beam splitter 208 and the lens 211 at the side of the first light output A with the suppressed spontaneous emitted light ingredient. The isolator makes it possible to suppress effects from the outside on the laser oscillation of the external resonator light source 205.
In FIG. 23, only the diffraction grating 209 for the laser oscillation by the external resonator light source 205 at the desired wavelength is shown. As opposed to this, the conventional external resonator light source shown in FIG. 24 is configured with the semiconductor laser diode 202, non-polarization beam splitter 208, diffraction grating 209, mirror 217, arm 218, and rotary shaft 219 arranged in the predetermined positional relationship illustrated and with the mirror 217 attached to the arm 218 rotating around the rotary shaft 219. This configuration is that of an external resonator light source of a Littmann layout. According to this configuration, it is possible to continuously set the resonance wavelength of the external resonator light source without mode hops and possible to obtain light with the suppressed spontaneous emitted light ingredient such as shown by the first light output A (light L1) by the non-polarization beam splitter 208. As explained above, the diffraction grating 209 is provided to become in a predetermined positional relationship with the rotary shaft 219. By rotating the rotary shaft 219, the arm 218 is moved as shown by the arrow 220 to change the angle formed by the arm 218 and the diffraction grating 209 and to change the position of the mirror 217 fixed to the arm. By this, it is possible to change the resonance wavelength of the external resonator light source.
Note that the laser light forming the light output B is emitted from the A-end 203 of the semiconductor laser diode 202. Further, the diffraction grating 209 produces the 0-th order light output C of the diffraction grating in addition to the light outputs A and B.
According to the configuration of the conventional wavelength tunable light source equipment explained above, light L1 with the suppressed spontaneous emitted light ingredient of the semiconductor laser diode 202 was taken out by providing a beam splitter, that is, the non-polarization beam splitter 208, inside the laser resonator of the external resonator light source 205. Therefore, there was the problem that the resonator loss of the external resonator light source 205 easily increased and became a factor destabilizing the laser oscillation or resonance.
Further, when taking out a lot of light output contributing to the laser oscillation from the laser resonator of the external resonator light source 205, the optical density in the external resonator light source 205 falls, the laser oscillation becomes unstable, and the laser oscillation itself ends up stopping. Therefore, it is difficult to obtain a large light output of light with the suppressed spontaneous emitted light ingredient of the semiconductor laser diode 202.
Further, according to the conventional wavelength tunable light source equipment 201, since the non-polarization beam splitter 208 is provided in the laser resonator of the external resonator light source 205, the relationship of intensity between the light L2 including the spontaneous emitted light ingredient from the semiconductor laser diode 202 and the light L1 including the suppressed spontaneous emitted light ingredient of the semiconductor laser diode 202 taken out from the non-polarization beam splitter 208 is largely governed by the laser oscillation conditions of the external resonator light source 205. The relationship of intensity of the light L1 and light L2 can be adjusted to some extent by adjusting the splitting ratio of the non-polarization beam splitter 208 and the reflectance of the A-end 203 of the semiconductor laser diode 202, but it is not possible to adjust it finely.
Normally, the output intensity of the light L1 with the suppressed spontaneous emitted light ingredient of the semiconductor laser diode 202 is a fraction or one order or more less than the output intensity of the light L2 including the spontaneous emitted light ingredient of the semiconductor laser diode 202. Therefore, it is difficult to make the intensity of the light L1 with the suppressed spontaneous emitted light ingredient of the semiconductor laser diode 202 larger than the output intensity of the light L2 including the spontaneous emitted light ingredient from the semiconductor laser diode 202.